This invention relates to cathode ray tubes, and more particularly relates to such tubes employing filters.
Cathode ray tubes employing filters are known. For example, it is known that the brightness of a color television projection display can be increased by the use of multilayer interference filters in the cathode ray display tubes, under the cathodoluminescent phosphor screen. Such filters tend to concentrate the light output of the tubes in the forward direction, resulting in more light being captured by the projection lenses, and consequently increased light output of the projection system. See, for example, U.S. Pat. Nos. 4,634,926 and 4,683,398, issued to Vriens et al., and assigned to U.S. Philips Corporation.
Such color projection systems rely on three separate cathode ray display tubes, one for each of the primary colors red, blue and green, to generate three separate monochrome images, which images are then superimposed on a projection screen to result in a full color image. To be effective, each tube must have a filter whose pass band characteristics match that tube's particular emission characteristics.
In addition to color projection systems, conventional direct view televisions can benefit from increased display brightness. However, the above-described use of interference filters customized to each of the primary color-emitting tubes is not applicable to direct view color TV's, because all of the colors are generated on one tube, and because there are no projection optics. Simply increasing the electron beam current to increase tube brightness generally leads to other problems, such as defocusing of the beam, increased energy consumption, increased doming, decreased cathode life, etc.